Raspberry PI vs PLC

For those of you who have already had experience with the Raspberry PI perhaps wonder, why the adoption of SBCs is common in the DIY sector but less popular in the industrial sector.

Of course, there is a reason for that. But we’ll get to that soon.

The good news first: You can already get a Raspberry PI Compute Module 3 with an industrial IP20 case from the manufacturer Kunbus.

Industrial hardened computer hardware is considerably more expensive than their counterparts in the consumer area. The reasons of the high price level for industrial equipment are the special requirements which are hard to archieve.

This is also the reason why the price of Kunbus RevPi Core 3 is higher by the factor 6 compared to a naked Raspberry PI 3.

Here is an overview of the usual requirements:

• 24/7 operation
• Electronic noise resistance (EN 61000–6–2 & EN 61000–6–4)
• High vibration and shock resistance (EN 60068–2–6 & EN 60068–2–27)
• Robust case (IP20 or higher)
• DIN-Rail mounting
• Proper operation in a wide temperature range (-25°C — +60°C)
• Proper operation in a wide humidity range (up to 95 % relative)
• Spare part supply for years
• Extensible (IO-system, fieldbus interface)

There are also some technological requirements which can be usually found only at industrial controllers:

• Real-time operation:
  Reading inputs, processing and writing outputs in <1 ms intervals.
• Built in USV (uninterruptible power supply):
  Crucial values can be stored persistent in specific registers in case of a power failure.
• Determinism:
  Exact temporal predictability what will happen and when.
• Online changes:
  Modification of the program during the running time of the program.
• Fieldbus interface:
  You may have heard of EtherCAT, EtherNet/IP or Profinet before for
  connecting peripherals.
• Higher output voltage and current:
  A standard digital output is able to supply a current in the
  range of 0.5 A — 2 A with 24 V.

Playing with the Raspberry PI or similar SBCs is nice, but when you need a robust and reliable controller for your application you should take a look on PLCs.

I defined 3 categories with a preselection of affordable PLCs from different manufacturers, each in a different price and functionality level. From a simple one with limited functionality to advanced systems for extended requirements. All controllers which I have selected have in common, that there are no additional expenses for the programming software. Finally, I will show how you can upgrade your Raspberry Pi that it will comply with some of the industrial requirements.

1. Programmable relays — Allen Bradley Micro 810

Allen Bradley Micro810

• 12 IO-Connectors
• Includes 4 relay outputs for switching up to 8 A
• Supply voltage: 120/240 V AC or 8–12 / 8–24V DC
• DIN-Rail mounting
• LCD-display upgrade available
• USB interface for programming
• 2.5 µs per basic instruction
• 2k program steps possible
• 2 kB for data
• PID control
• Analog input with 10 bit resolution
• 32 and 64 bit floating point precision
• Power consumption: 5 W
• Programmable by Connected Components Workbench
  (structured text, function block and ladder diagram)

This controller enables you to setup simple automation applications: Switching engines on certain conditions, programming step sequences or using its PID capabilities. Keep in mind that this controller has no RTC. For date and time related applications you need the next better class.

The price for such controllers starts at about 75 €

2. Controller with onboard IO and fieldbus interface

Schneider Modicon M221

• 9 inputs, 7 relay outputs (2 A)
• Extensible by IO-cartridges
• Extensible by a 2 GB SD-card
• Battery buffered RTC
• 50 words for persistent values
• Serial interface (RS232/RS485, RJ45 connector)
• Ethernet interface (100Mbit)
• Modbus TCP and EtherNet/IP compatible
• 0.2 µs per basic instruction
• 256 kB for user programs
• 256 kB for variable data (RAM)
• Diagnose LEDs
• Programmable with SO Machine Basic

3. Embedded industrial PCs

Beckhoff CX9020

With an IPC like the Beckhoff CX9020, you are able to implement almost every application. Functionalities which aren’t available out of the box can easily be added by the help of the numerous interfaces (e.g. USB Wifi dongle).

• 1 GHz Cortex A8 ARM CPU (1 Core)
• 1 GB DDR3-RAM
• 128 kB NOVRAM (for persistent values)
• 2x Ethernet on switched RJ45 interfaces (10/100 Mbit)
• 4x USB 2.0
• Optional: RS232, Profibus or CANopen
• Windows Embedded Compact 7 (User mode programming)
• Extensible by E-bus or K-bus terminals (automatic recognition)
• DVI connector for monitors
• Programmable with TwinCAT 3

Note that this controller has no on-board IO. It can be extended by a whole bunch of EtherCAT terminals or K-Bus terminals which must be ordered separately. An over-voltage event which destroys an digital or analog input would only cause the exchange of a single IO-terminal instead of replacing the whole controller. The high performance of the E-bus interface makes this controller also suitable of measuring tasks in test stands or laboratories.

You can get such a controller for about 650 €.

Upgrade for the Raspberry Pi

• Raspberry PI 3 Model B: 30 €
• CoDeSys runtime for 61131–3 programming: 50 €
• Power supply: 120–230 V AC → 5 V DC: 25 €
• DIN-rail housing: 20 €

Together that makes 125 €

A direct comparison at this point wouldn’t be fair. The CoDeSys runtime designed for the Raspberry PI has no real time capabilities and is primary designed for testing and prototyping.

The main use cases of an industrial PLC is too far apart from that of a Raspberry PI. But to get an idea of the performance level of a Raspberry PI, here is a short extract of the data sheet (model 3B):

• Quad Core 1.2 GHz 64 Bit ARM CPU
• 1 GB RAM
• Wifi & Bluetooth on-board
• 40x GPIO (0 - 5V level)
• 4x USB 2.0
• HDMI interface

I will not try to explain what you can do with it. You can find many great projects out there which are making more or less use of the Raspberry PIs functionality.

Conclusion

There are many ways to get an reliable controller for 24/7 automation applications. If you need a controller for a real industrial application, there is almost no way around a genuine PLC. For prototyping or DIY projects, a Raspberry PI with some upgrades will mostly satisfy all of your requirements. All in all, the required functionality and the available budged will make the decision in one direction or the other.